Objectives We studied the cotton seed qualities of the four seasonal bolls and their responses to nitrogen fertilizer and mepiquat chloride (DPC) application, to provide a scientific supports for efficient production of high quality cotton seeds.

Methods A two-factor split-plot design field experiment was conducted from 2015 to 2017 in the Yellow River beach area of Zhengzhou. The main plots were assigned to N rates, namely, 0, 225 and 450 kg/hm², representing no N control (N0), normal (N2) and excessive N rate (N2); The subplots were DPC dosages, namely, 0, 75 and 150 g/hm² (D0, D1, and D2), representing no DPC, normal and excessive DPC dosages, respectively. The cotton bolls setting in pre-summer, summer, early-autumn and late-autumn respectively were picked for the determination of seed quality. The investigated sowing quality indexes included seed index (SI), germination potential (GP) and germination rate (GR), and the nutritional quality indexes included total protein, crude fat, total starch, soluble protein and soluble sugar.

Results Significant differences in sowing and nutritional qualities of seeds from four seasonal bolls were observed. The SI, GP, and GR of cotton seeds from summer and early-autumn bolls were similar, but significantly higher than those from pre-summer and late-autumn bolls. The cotton seeds of summer and early-autumn bolls contained significantly higher total protein and soluble protein, but lower total starch and soluble sugar than the seeds from pre-summer and late-autumn bolls, all the cotton seeds of the four seasonal bolls contained similar crude fat. The application of nitrogen fertilizer and DPC showed significant promotion on the sowing and nutritional qualities of four seasonal boll seeds. Compared with N2 on average, N1 treatment was recorded similar SI, but higher GP and GR; significantly higher total protein, soluble protein and crude fat, but lower total starch and soluble sugar. Compared with D2 on average, D1 treatment was recorded lower SI, but higher GP and GR; significantly higher total protein, soluble protein and crude fat, but lower total starch and soluble sugar. Among all the combined treatments, N1D1 showed the highest promotion effect, the seed GP, total protein, soluble protein and crude fat were 3.35%–40.86%, 3.80%–43.01%, 13.75%–54.86%, and 1.11%–13.48% higher than those in the other combined treatments, respectively. The GP and GR of cotton seeds were positively correlated with total protein, crude fat and soluble protein, and negatively correlated with total starch and soluble sugar content.

Conclusions In the cotton-growing region of Yellow River valley, the sowing and nutritional qualities of cotton seeds form summer and early-autumn bolls are obviously superior to those from pre-summer and late-autumn bolls. Reasonable combination of nitrogen fertilizer and DPC could increase the total protein, crude fat and soluble protein content, reduce the total starch and soluble sugar content, and improve the GP and GR of seeds. Suitable nitrogen fertilizer rate (225 g/hm²) and DPC dosage (75 g/hm²), and picking summer and early-autumn bolls are recommended to improve the quality of cotton seeds.